I would go to any marina or store that caters to the boating public and
buy some "Spar" or "Marine" varnish for your mice signs.
Spar (aka: Marine) varnish is intended to withstand exposure to the
elements as it's original purpose was in coating and sealing the exposed
wooden spars on tall ships. It's most important when protecting any
wood from the elements to treat the end grain of the wood. This is
because wood absorbs moisture through it's end grain 15 times faster
than it absorbs moisture across it's grain. So, weathering of wood
usually is the result of the absorbtion of rain water or snow melt into
the end grain of the wood. Where you have plywood, you have end grain
along the entire periphery of the plywood.
Oil based coatings typically will yellow with age when used in a place
with dim or minimal lighting. However, in direct or indirect sunlight,
the Sun's rays will remove that acquired yellow discolouration. So,
while you might a yellowish tint on the paintings from the spar varnish
itself, any further yellowing will be eliminated once the paintings are
exposed to direct or indirect sunlight for a few weeks. It would be
best to arrange the paintings somewhere in your yard that receives a
fair bit of sunlight for a few weeks before putting them on display for
the general public to see.
Museum curators, who typically have far more paintings (done in oil)
than they could ever put on display at one time will have sunny rooms in
the museum where they can put oil based paintings that they intend to
put on display in a few weeks. That way, the public sees the painting
the way the painter painted it, not in it's yellowed condition after
spending years in dimly lit storage rooms.